The 6-foot-10 Karlovic, the tallest player in the history of the ATP World Tour, ousted the Wimbledon champion Djokovic in 6-7 (2-7), 7-6 (8-6), 6-4 fashion in 2 hours, 15 minutes on the hardcourts at Khalifa International Tennis Complex.

"Dr. Ivo" swatted 21 aces, including four straight to close out the final game of the match. The Croat recorded the only break of serve in the bout in the seventh game of the final set.

"It was unbelievable," Karlovic said during his on-court interview following the match. "When I arrived here, I didn't expect that one. He wasn't able to show his full range because it was windy. I just focused on my game and in the end I was a little luckier."

Karlovic is now 2-1 lifetime against Djokovic, including a perfect 2-0 on hardcourts. He also claimed his second career victory over a world No. 1, with his previous win over a top-ranked player coming at the expense of Roger Federer in Cincinnati in 2008.